Photoacoustic imaging is an emerging hybrid imaging technology providing optical contrast with high spatial resolution. Nanosecond or picosecond laser pulses fired into tissue launch thermo-elastic-induced acoustic waves which are detected and reconstructed to form high-resolution images. Photoacoustic imaging has been developed into multiple embodiments, including photoacoustic tomography (PAT), photoacoustic microscopy (PAM), optical-resolution photoacoustic microscopy (OR-PAM), and array-based PA imaging (array-PAI). In photoacoustic tomography (PAT) signals are collected from multiple transducer locations and reconstructed to form a tomographic image in a way similar to X-ray CT. In PAM, typically, a single element focused high-frequency ultrasound transducer is used to collect photoacoustic signals. A photoacoustic signal as a function of time (depth) is recorded for each position in a mechanically scanned trajectory to form a 3-D photoacoustic image. The maximum amplitude as a function of depth can be determined at each x-y scan position to form a maximum amplitude projection (MAP) C-scan image. Photoacoustic microscopy has shown significant potential for imaging vascular structures from macro-vessels all the way down to micro-vessels. It has also shown great promise for functional and molecular imaging, including imaging of nanoparticle contrast agents and imaging of gene expression. Multi-wavelength photoacoustic imaging has been used for imaging of blood oxygen saturation, by using known oxy- and deoxy-hemoglobin molar extinction spectra.
In traditional photoacoustic imaging, spatial resolution is due to ultrasonic focusing and can provide a depth-to-resolution ratio greater than 100. In OR-PAM, penetration depth is limited to ˜1 mm in tissue (due to fundamental limitations of light transport) but resolution is micron-scale due to optical focusing. OR-PAM can provide micron-scale images of optical absorption in reflection-mode, in vivo, something that no other technique can provide. OR-PAM is capable of imaging blood vessels down to capillary size noninvasively. Capillaries are the smallest vessels in the body and so much crucial biology occurs at this level, including oxygen and nutrient transport. Much can go wrong at the capillary level too. In cancers, cells have an insatiable appetite for oxygen and nutrients to support their uncontrolled growth. They invoke a range of signaling pathways to spawn new vessels in a process known as angiogenesis and these vessels typically form abnormally. Tumors are often highly heterogeneous and have regions of hypoxia. Photoacoustic imaging has demonstrated the ability to image blood oxygen saturation (SO2) and tumor hypoxia in vivo.
In most photoacoustic and ultrasound imaging systems, piezoelectric transducers have been employed, in which an ultrasound coupling medium such as water or ultrasound gel is required. However for many clinical applications such as wound healing, burn diagnostics, surgery, and many endoscopic procedures physical contact, coupling, or immersion is undesirable or impractical.
The detection of ultrasound in photoacoustic imaging has, until recently, relied on ultrasonic transducers in contact with the biological tissue or an ultrasonic coupling agent both of which have major drawbacks as described above. Some detection strategies to solving the non-contact optical interferometric sensing problems associated with photoacoustic imaging have been reported.
Optical means of detecting ultrasound and photoacoustic signals have been investigated over a number of years; however, to date no technique has demonstrated practical non-contact in vivo microscopy in reflection mode with confocal resolution and optical absorption as the contrast mechanism.
Most previous approaches detected surface oscillations with interferometric methods. Others used interferometry to observe photoacoustic stresses, including optical coherence tomography (OCT) methods. These methods offer potential sensitivity to the scattered probe beam phase modulations associated with motion of scatterers, subsurface and surface oscillations, as well as unwanted vibrations. They are also sensitive to complex amplitude reflectivity modulations.
One example of a low-coherence interferometry method for sensing photoacoustic signals was proposed in U.S. pregrant publication no. 2014/0185055 to be combined with an optical coherence tomography (OCT) system, resulting in 30 μm lateral resolution.
Another prior art system is described in U.S. pregrant publication no. 2012/0200845 entitled “Biological Tissue Inspection Method and System”, which describes a noncontact photoacoustic imaging system for in vivo or ex vivo, non-contact imaging of biological tissue without the need for a coupling agent.
Other systems use a fiber based interferometer with optical amplification to detect photoacoustic signals and form photoacoustic images of phantoms with acoustic (not optical) resolution. However these systems suffer from a poor signal-to-noise ratio, other contact-based photoacoustic systems offer significantly improved detection capabilities, in vivo imaging was not demonstrated, and optical-resolution excitation was not demonstrated.
Industrial laser ultrasonics has used interferometry to detect acoustic signatures due to optical excitation of inanimate objects for non-destructive testing. This approach has been adapted to detect ultrasound ex vivo in chicken breast and calf brain specimens, however, optical-resolution focusing of the excitation light was not examined.
Laser Doppler vibrometry has been a powerful non-contact vibration sensing methodology, however, weak signal-to-noise and poor image quality have proven to be a limitation when sensing deep-tissue signals from broad-beam photoacoustic excitation.
Similarly, Mach Zehnder interferometry and two-wave mixing interferometry have been used previously for sensing photoacoustic signals. However many such techniques still require direct contact or fluid coupling; have not offered in vivo studies or optical resolution for phantom studies.
The non-interferometric photoacoustic remote sensing (NI-PARS) is fundamentally different from other approaches for detection ultrasound/photoacoustic signals. The system takes advantage of a pulsed excitation beam co-focused and co-scanned with an interrogation beam. The detection mechanism is based on a non-interferometric sensing. Rather than detecting surface oscillations, pressure-induced refractive-index modulation resulting from initial pressure fronts can be sampled right at their subsurface origin where acoustic pressures are large. The non-interferometric nature of detection along with the short-coherence lengths of the interrogation laser preclude detection of surface- and sub-surface oscillations to provide only the initial pressure signals.